Two of the dead rhinos came from South Africa's Kruger National Park and two were born in captivity. (Getty Images)

(Newser)
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Rhino experts from around the world have been stumped by an illness killing off white rhinos at an Australian zoo. Four white rhinos have died at the New South Wales zoo over the last few weeks. All of them died suddenly after displaying neurological problems like stumbling, the Age reports. The zoo, which runs a breeding program, has placed its remaining three white rhinos in quarantine.

"So far the investigation has ruled out exposure to toxins, bacterial infection, snake venom and organ failure as causes of death," the zoo said in a statement. Possible viral causes are also being probed, but the most likely ones have already been ruled out. "Obviously, the rhino keepers and veterinary staff know and care for every individual in the herd, so this has been a huge shock. We're all very sad and supporting each other through this difficult time," said the zoo's manager.